The present invention relates to a radio base station apparatus and radio communication method used in a digital radio communication system.
In a digital radio communication system, in the case where transmission with controlled directivity is performed on a downlink (downlink adaptive array antenna transmission), the number of sectors, which means areas where transmission/reception is performed with same directivity, is expected to reduce. That is, it is possible to achieve an effect similar to sectorization by carrying out transmission with directivity, which transmits data with beams focused on a specific direction through signal processing, and therefore applying transmission with directivity is believed to reduce the number of physical sectors.
Thus, the number of channels (number of user terminals) that can be accommodated per sector will increase by reducing the number of sectors through effects of reducing the number of sectors and applying an adaptive array antenna, that is, by enabling transmission with directivity to be performed through signal processing. When the number of channels that can be accommodated within a sector increases, concerns rise that there will inevitably be a shortage of spreading codes under a same scramble code on the downlink. To resolve this shortage of spreading codes, it is anticipated that a plurality of scramble codes will be used within the same sector.
Conventionally, when a plurality of scramble codes is used within the same sector, the number of scramble codes is simply increased to resolve the shortage of spreading codes, and in this way scramble codes are assigned regardless of directivity of each channel. In such a case, as shown in FIG. 1, there is a possibility that different scramble codes #0 to #2 will be used among a plurality of channels in a same direction.
Originally, a same scramble code is used in a sector and transmission from a base station is carried out in synchronization therewith, and therefore orthogonality with no cross-correlation interference among codes can be expected. However, if scramble codes #0 to #2, which vary among a plurality of channels in a same direction, are used, the orthogonality can no longer be guaranteed (expected).
Thus, compared to a case where no adaptive array antenna is used, that is, the same scramble code is used for all channels within a sector, there will be a problem in reception at each communication terminal that cross-correlation interference from other channels occurs and the reception characteristic deteriorates.
It is an object of the present invention to provide a radio base station apparatus and radio communication method capable of reducing cross-correlation interference among a plurality of channels when a plurality of scramble codes is used within a same sector.
A subject of the present invention is to assign a same scramble code to channels with similar (not necessarily the same) directivities in a system that uses transmission with controlled directivity on a downlink (adaptive array antenna), thereby improve orthogonality among a plurality of channels in a same direction and reduce cross-correlation interference from other channels during reception from each communication terminal.